The major component of the mammalian heart's contractile apparatus, the myosin heavy chain, exists as two distinct isoforms, V1 and V3, encoded by a-MyHC and b-MyHC, respectively. The relative amounts of the two isoforms vary, depending upon the developmental stage, the particular cardiac compartment, the animal and the physiological status of the heart. The long-term objectives of this proposal are to understand the functional consequences of isoform shifts in the context of whole heart function and the long-term physiological effects on the heart.